Project Summary

Project Abstract:In general, the Hamburg Wheel Tracking Test (HWTT) has a proven history of identifying hot-mix asphalt (HMA) mixes that are moisture susceptible and/or prone to rutting. However, with the record summer temperatures of the recent years, several rutting failures have occurred with HMA mixes that had passed the Hamburg test in the laboratory. These failures occurred mostly in high shear locations, in particular with slow moving (accelerating/decelerating) traffic at controlled intersections, in areas of elevated temperatures, heavy/high traffic loading, and/or where lower PG asphalt-binder grades have been used. Earlier TxDOT studies had raised concerns about the Hamburg test in that it is run at one temperature (50 °C) and it provides high confinement to the test sample. These studies also demonstrated that the repeated load permanent deformation (RLPD) test has a better correlation than the Hamburg to field rutting performance. The RLPD test also provides material properties, which can be used in mechanistic-empirical (M-E) pavement thickness design procedures. However, the current RLPD test set-up is relatively complex and not readily applicable for routine use. This makes it impractical to be used for routine HMA mix screening/acceptance and/or M-E design.

As a supplement to the Hamburg test, research is therefore needed to develop a simpler and less time consuming shear resistance and permanent deformation (PD)/rutting test that is also cost-effective, repeatable, and produces superior results in terms of correlation with field rutting performance. In particular, such a test should have the potential to discriminate HMA mixes for application in high shear stress areas (i.e., intersections) as well as being an indicator of the critical temperatures at which a given HMA mix, with a given PG asphalt-binder grade, becomes unstable and more prone to rutting and/or shear failure. The scope of work to accomplish these objectives will include the following activities:

Data search and literature review,

Computational modeling and shear stress-strain analysis,

Modification and/or development of rutting-shear tests,

Sensitivity and statistical analyses of the test methods,

Correlation with field data and development of test procedures/specifications, and

Test demonstration with a case study.

An extensive data search on the current rutting-shear tests along with comprehensive shear stress-strain analyses will be conducted in the first months of the project. Plans for modifying the existing tests and/or developing new test methods will be developed and after TxDOT approval, will be implemented by the research team. Procedures and guidelines for implementing the modified and/or new tests will also be submitted to TxDOT at the end of the study.

Project Objectives:The work plan presented herein was devised to address the main technical objectives of the research project, namely to:

1) To conduct computational modeling and shear stress-strain analysis pavement structures so as to determine the critical zones of shear deformation and the threshold for the maximum failure plastic strains.

2) To improve and/or modify the existing rutting-shear protocols.

3) To design and develop a simple and practical HMA shear resistance and PD/rutting test to supplement the Hamburg test.

4) To conduct sensitivity analysis and field correlation of the new test methods as compared to the current HWTT on a range of Texas HMA mixes and field test sections, including rubber/polymer modified, RAP, and RAS materials

5) To formulate guidelines and assist in the implementation of the new test methods.

Task 6: Sensitivity Evaluation of the Tests Developed from Tasks 4 and 5

Task 7: Correlation of the Developed Lab Tests to Field Data

Task 8: Development of Guidelines and Test Procedures/Specifications

Task 9: Case Study – Test Demonstration and Workshop

Task 10: Report Writing

Implementation of Research Outcomes:Traditionally run at one test temperature (122°F), the Hamburg Wheel Tracking Test (HWTT) has a proven history of identifying hot-mix asphalt (HMA) mixes that are moisture susceptible and/or prone to rutting. However, with the record summer temperatures of the recent years, several shear and rutting failures have occurred with HMA mixes that had passed the HWTT in the laboratory; mostly in high shear locations, in particular with slow moving (accelerating/decelerating) traffic at controlled intersections, stop-go sections, in areas of elevated temperatures, heavy/high traffic loading, and/or where lower PG asphalt-binder grades have been used.

As a supplement to the HWTT, this two-year study is being undertaken to develop a simpler and less time consuming shear resistance and permanent deformation (PD)/rutting test that is also cost-effective, repeatable, and produces superior results in terms of correlation with field rutting performance. In particular, such a test should have the potential to discriminate HMA mixes for application in high shear stress areas (i.e., intersections) as well as being an indicator of the critical temperatures at which a given HMA mix, with a given PG asphalt-binder grade, becomes unstable and more prone to rutting and/or shear failure.

In line with these objectives, this interim report documents the research work completed in Year-1 of the study, namely: a) data search and literature review; b) computational modeling and shear stress-strain analysis; c) comparative evaluation of the Asphalt Mixture Performance Tester (AMPT) and the Universal Testing Machine (UTM); d) comparative evaluation of the Flow Number (FN), Dynamic Modulus (DM), and Repeated Load Permanent Deformation (RLPD) tests relative to the HWTT test method.

Impacts/Benefits of Implementation:After completion of year two of this study, procedures and guidelines for implementing the modified and/or new tests will also be submitted to TxDOT.